A cyclic-structure-containing branched glucan produced by allowing a branching enzyme to act on a starch has following excellent properties: it has no reducing end; their solubility in water is higher as compared with those of natural unmodified starch; viscosity of its solution is low; and retrogradation which is observed for natural unmodified starch is difficult to occur. The cyclic-structure-containing branched glucan has been developed and utilized as a useful substitute substance for starch.
The cyclic-structure-containing branched glucan can be produced by heating starch such as waxy corn starch to a temperature that is the gelatinization starting temperature or higher of the starch to dissolve the starch, then, cooling the starch solution to a temperature at which a branching enzyme can react therewith and, thereafter, adding a branching enzyme to this starch solution to allow to act thereon (Patent Document 1). However, in this method, the starch solution becomes a rice cake state during cooling, and flowability cannot be ensured. For example, Example 1 in paragraph 0119 of Patent Document 1 describes an experiment in a small scale at a laboratory level, in which a small amount of waxy corn starch (5 g) is suspended in 40 ml of a buffer solution to gelatinize the starch at 100° C. and, after allowing it to cool to 50° C., a branching enzyme is allowed to act thereon. Although the description of a detailed procedure is not given in paragraph 0119 of Patent Document 1, a working step which is not industrially practical, such as inserting a spoon into a reaction tube to forcibly stir a starch solution having no flowability (i.e., gel), is necessary for mixing an enzyme and a substrate in such an experiment at a laboratory level, and actually, the inventors of the subject matters described in Patent Document 1 have performed the work of forcible stirring when they performed the experiment in paragraph 0119 thereof. It is also clear from FIG. 2 of TAKATA et al., Journal of fermentation and bioengineering Vol. 84, No. 2, 119-123, 1997 that such a high concentration starch solution has no flowability. For this reason, if it is attempting to industrially produce a cyclic-structure-containing branched glucan by this method, since a starch solution has no flowability, clogging of a pipe for transporting the starch solution to a reaction vessel is caused so that the starch solution for performing an enzymatic reaction cannot be transported to the reaction vessel.
For this reason, when a cyclic-structure-containing branched glucan is industrially produced, the following method is adopted (Patent Document 2): feeble α-amylase is added to a suspension of starch, the suspension is heated to a temperature that is the gelatinization starting temperature of the starch or higher, to proceed dissolution of the starch and low molecularization of the starch due to partial hydrolysis of the starch simultaneously, and thus, a starch-gelatinized liquid having ensured flowability is prepared and, thereafter, a branching enzyme (also described as BE in the present specification) is added to this starch-gelatinized liquid to allow to act thereon. However, in such a method of using both of thermal gelatinization and hydrolysis, hydrolysis of the starch occurs and, as a result, a non-cyclic glucan having a reducing end is generated. For this reason, in this method, only a mixture of a cyclic-structure-containing branched glucan having no reducing end and a non-cyclic glucan having a reducing end could be obtained. The cyclic-structure-containing branched glucan having no reducing end will not be colored even when heated with an amino acid or a protein, but the non-cyclic glucan having a reducing end has a defect that it will be colored when heated with an amino acid or a protein. Furthermore, it is extremely difficult to industrially isolate the cyclic-structure-containing branched glucan having no reducing end, from this mixture.
A method for producing a cyclic-structure-containing branched glucan having no defect as described above and no reducing end, at an industrial scale, with high purity has not been known so far. That is, a method for producing a cyclic-structure-containing branched glucan, wherein a glucan having a reducing end is not generated as a byproduct, that is, a method for industrially producing a cyclic-structure-containing branched glucan without using α-amylase, has not been known.
In addition, Patent Document 3 describes a method for producing a highly branched glucan using raw starch. However, the purpose of Patent Document 3 is producing a highly branched glucan, and the highly branched glucan has a reducing end, and the purpose of Patent Document 3 is not producing a glucan having no reducing end. Further, a reducing sugar is included in the product described in Patent Document 3.